A central topic of the novel mechanisms presented herein is resource allocation, also referred to as scheduling in cellular communication networks, with EPS/LTE (Evolved Packet System/Long Term Evolution) as the main example. Scheduling in cellular communication networks involves allocating transmission resources for communication pertaining to a certain wireless terminal. These transmission resources may be uplink (i.e. in the direction from the wireless terminal to a network node, e.g. a base station) or downlink (i.e. in the direction from a network node, e.g. a base station (BS), to a wireless terminal) transmission resources.
In EPS/LTE transmission resources consist of parts of a time-frequency grid and a wireless terminal is commonly referred to as a user equipment (UE). The smallest transmission resource unit in EPS/LTE is called a resource element, which are grouped into resource blocks. A resource block consists of 12 subcarriers of 15 kHz each in the frequency domain and a slot of length 0.5 ms in the time domain. Pairs of slots are further grouped together to form 1 ms subframes. Each slot consists of 7 resource elements (or 6 if an extended cyclic prefix is used), each containing an OFDM symbol including cyclic prefix. Hence, each resource block consists of 84 resource elements (or 72 if the extended cyclic prefix is used)
The basis for uplink and downlink scheduling in cellular communication networks, like EPS/LTE is so-called dynamic scheduling. In dynamic scheduling, a new scheduling decision is taken in each schedulable timeslot, which allows for full flexibility in terms of the resources used. In EPS/LTE such a schedulable timeslot is a 1 millisecond long so called subframe. By use of dynamic scheduling, large variations in the amount of data to transmit can be handled, at the cost of scheduling decisions being sent on a downlink control channel, e.g. the PDCCH (Physical Downlink Control Channel) in each schedulable timeslot. In EPS/LTE (Evolved Packet System/Long Term Evolution), transmission resources, in terms of time and frequency, are typically allocated by a scheduler in an eNB for each downlink or uplink transmission. The allocated transmission resources are then indicated on the PDCCH by use of a scheduling assignment in case of downlink (also called downlink resource assignment), or, an uplink grant in case of uplink. Uplink resource allocations, i.e. uplink grants, are typically preceded by a scheduling request from the UE, while downlink resource allocations are triggered by pending downlink data. Associated with each resource allocation, and indicated on the PDCCH, is a modulation and coding scheme (MCS), which is selected by the eNB and which is used for the data transmission on the transmission resources that are allocated by the resource allocation. The transport block size is not explicitly indicated in the resource allocation, but may be derived from the combination of the size of the allocated resources, i.e. the number of resource blocks, and the MCS.
Scheduling in LTE is performed on a subframe basis, and in each subframe the available bandwidth may be allocated to one or more UEs. In the frequency domain the allocations have to adhere to resource block boundaries. The smallest possible allocation is thus two resource blocks, one in each slot of a subframe. When data is transmitted using the allocated resources, the MCS and transport format are chosen such that they match the size of the allocated resources and the bits to be transmitted are mapped to the resource elements (and OFDM symbols) of the allocated resource. In order to enable coherent detection at the receiver (i.e. the eNB), a transmitting UE includes a cell specific Demodulation Reference Signal (DMRS) interleaved with the data. Specifically, the DMRS is transmitted in the fourth (or third if an extended cyclic prefix is used) OFDM symbol of each slot, i.e. twice every subframe, across the entire allocated transmission resource, i.e. on all subcarriers of the allocated resource blocks. From 3GPP release 11 a DMRS may be made UE specific based on a combination of DMRS sequence and phase rotation of the sequence.
For downlink transmissions, the cell specific reference signals are typically all reference signals that are needed. However, optionally, the eNB may insert UE specific reference signals in the third or sixth OFDM symbol (when a normal cyclic prefix is used) every second subcarrier of a resource block transmitted to a UE.
In a currently popular vision of the future development of the communication in cellular networks, huge numbers of, mostly, small autonomous devices become increasingly important. These devices are assumed not to be associated with humans, but rather to be sensors or actuators of different kinds, which communicate with application servers, which configure the devices and receive data from them, within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication and the devices may be denoted machine devices (MDs). In the 3GPP standardization the corresponding alternative terms are machine type communication (MTC) and machine type communication devices (MTC devices), with the latter being a subset of the more general term user equipment, UE. In terms of numbers MTC devices will, according to the vision, dominate over human users, but since many of them will communicate very scarcely, their part of the traffic volume will be much smaller than their part of the “user” population.
With the nature of MTC devices and their assumed typical uses follow that they will often have to be very energy efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries.
For such energy deprived devices the traffic is characterized by small, more or less infrequent transactions—often delay tolerant, which will result in a large signaling overhead. Hence, reducing the signaling overhead is an important means to facilitate for such devices to function efficiently, with a long battery lifetime, using a wireless communication network.